How do I retain drill-down reporting with a null value in a data set

Summary: Hello. I have a dashboard which has several pies on it that draw data from talent review categories. I need to enable drill-down person reports from the pies for null ratings data.

Content (required): The pies use counts and ratings to build the display, and includes null values which means the person wasn't rated on that particular talent category. When a user clicks on the piece of the pie for the rating they wish to see detail for, a report pops up in a new window that displays the group of people the selected rating applies to. Because the value for one of the pie pieces is null, I'm finding I either can't name the rating (e.g., 'Unrated') in the legend or I can't do any drill-down report display